High vacuum chamber



Aug. 20, 1963 H. R. SMITH, JR

HIGH VACUUM CHAMBER Filed. Nov. 9, 1959 FIG'Z BY OK flrri i/i UnitedStates Patent I 3,101,169 HlGH VACUUM CHAMBER Hugh R. Smith, In,Oakland, Caliil, assignor to Staufier Chemical Company, New York, N.Y.,a corporation of Delaware Filed Nov. 9, 1959, Ser. No. 851,878 7 Claims.(El. 23tl-lill) The present invention relates in general to an improvedpumping geometry for vacuum chambers, and more particularly to a chamberstructure having maximized pumping area defining the chamber forattaining high pumping speeds to maintain a high vacuum therein.

In various processes and apparatus it is necessary to evacuate liberatedgas from reaction chambers and to establish and. maintain a vacuumtherein. This is normally accomplished by the utilization ofconventional vacuum pumps, such as diffusion pumps, or the like, whichmay also be combined with backing pumps to attain the desired degree ofvacuum. In certain circumstances, this conventional solution is found tobe unsatisfactory because of physical space limitations. Withoutlimitation, an ex-.

ample of the foregoing is found in electron-beam furnaces utilized inthe purification and casting of metals. In at least certainelectron-beam processes for casting and purifying metals, there areemployed high-vacuum techniques for the purpose of removing gasesliberated from metals melted by electron-beam bombardment. In order forsuch processing to proceed with maximum efficiency, it is highlyimportant for relatively large quantities of liberated gas to be veryrapidly removed from the processing region, i.e., from the directvolume: of melted metal. While it is theoretically possible to providedesired pumping speeds by the addition of a suitable number ofconventional vacuum pumps about the reaction or furnace chamber, apractical difiiculty is encountered in the physical space limitationspresent. it is well known that conventional vacuum pumps havesubstantial physical sizes, which will be noted to be related to thepumping speed of such pumps. In order to attain a very pumping speedoperable in a limited volume, it necessary to provide a large pluralityof large vacuum pumps in direct communication with such volume orregion. It has. been found that certain physical difficulties areinvolved in this multiplication of pumping means, for the physical sizeof the pumps places an upper limit on the number of pumps which can beplaced closely adjacent to any limited volume.

The physical disposition of vacuum pumps at any substantial distancefrom a limited volume to be maintained at a particular vacuum has beenfound to be ineffectual to prevent serious pressure excursions in thevolume of interest. These effects are particularly deleterious when sionof the enclosure, or greater. Under these conditions the geometricalimpedance to the flow of the gases and vapors into the pumps will be ata maximum. Thus the pumps must be very closely coupled through thewidest possible orifices to the vacuum enclosure to be elfective. Thenet pumping speed is related to the area over which pumping isaccomplished. It thus follows that no matter how efficient vacuum pumpsmaybe, there may yet remain a serious difficulty in removing largevolumes of gas from relatively limited regions to maintain asubstantially constant vacuum in such region. This particular problem,(while present in various processes, directly arises inthe above-notedexample of electron-beam casting of metals in vacuum wherein verysubstantial quantities of gas are almost instantaneously liberated atcertain points in the process, depending upon the purity 3,ll,l69Patented Aug. 20, 1963 and to sustain a pumping speed of more than 30,000 liters per second, further improvements in the vacuum and pumpingspeed for such applications are beneficial, if not requisite, toadditional advances in the field of vacuum metallurgy.

The present invention provides a chamber geometry highly suited to theestablishment of very large pumping speeds whereby a chamber volume ofrelatively limited size may be maintained at a very low pressure despitesporadic bursts of gas from materials processed therein. A chamberstructure highly suited for a furnace chamber to be used inelectron-beam processing of metals in vacuum is herein provided with amaximized defining area which is available for the pumping of gas. Indistinction 'to conventional vacuum. chambers wherein a plurality ofports are commonly provided in the walls thereof communicating withvacuum pumps, the present invention provides a chamber which is itselfdefined by pump inlets. attain a pumping area that is as large aspercent of the total wall area of a chamber to be evacuated. With thischamber structure it is thus possible to attain pumping speeds of theorder of hundreds of thousands of liters per second. With such pumpingspeeds it has been found possible tov maintain chambers of limitedvolume at operating pressures of the order of 0.01 to 0.10 micron whilevery substantial quantities of gas are liberated within the chamber. Inthe field of vacuum metallurgy wherein electron-beam melting isemployed, for example, it is possible in accordance with the presentinvention to materially increase the melting rate of metal beingpurified while at the same time substantially eliminating the evolutionof gas bursts in the chamber. The tremendous pumping speed availablewith the present invention serves to remove gas evolved during theprocess of casting and purification so rapidly that almost no pressurevariations occur within the processing volume, and consequently amaterially improved processing is possible. By the utilization of thepresent invention it is also possible to reduce the overall height offurnace chambers of the type herein discussed, for the physical size ofsuspendeddilfusion pumps which normally dictate the minimum chamberdimensions are no longer ruling. As particularly applied in whichmaterial is being melted to provide pumping for theevacuation of suchregion.

It an object of the present invention to provide an improved geometryfor maximized evacuation of suc chamber.

It is another object of the present invention to provide an improvedstructure for a furnace chamber wherein a maximum available wall area isemployed to evacuate the chamber.

It is yet another object of the present invention to provide an improvedfurnace dhamber structure for an electron-beam furnace.

It is another object of the present invention to provide an improvementin the vacuum casting of metals by electron-beam bombardment through theenhanced evacuation of the reaction region therein.

Various other possible objects and advantages of the present inventionwill become apparent to those skilled in the art from the followingdescription of a single preferred embodiment of this invention. Nolimitation is intended by the terms of the following disclosure, andinstead reference is made to the appended claims for a In accordanceherewith it is possible to.

precise delineation of the true scope of the present invention.

The invention is illustrated as to a single preferred embodiment thereofin the accompanying drawings wherein: 7 FIG. 1 is a plan view of afurnace chamber with the top thereof removed.

FIG. 2 is a sectional view of a furnace chamber taken in the plane 22 ofFIG. 1.

The present invention in 'ef provides a vacuum cham ber structureincluding a pumping wall in large part defining the chamber. Thispumping wall preferably extends about the chamber and is [formed of aplurality of pumps having a high ratio of inlet openings to physicalstructure at the inlet end so that the wall is mainly composed ofpumping area. A maximized pumping area for the chamber is hereinattained by the utilization of pumps having optimized inlet openings. Ofparticular interest in this respect are strip pumps of quite simple andstraightforward construction and which include but a minimum ofstructural elements tending to restrict the pump inlet. These pumps arefound to be readily combined into a multiple pumping unit that isadmirably suited to define a chamber or the like that is intended to beevacuated. By the provision of a maximized pumping area defining achamber to be evacuated, the present invention provides a maximumpumping speed for such chamber to thereby most efiiciently andrapidlyremove gases which may be evolved in such chamber. In particular, thegeometry of the present invention is admirably suited to the rapidremoval of large quantities of gas from a very limited volume. It ispossible in accordance herewith to locate an exteremely large pumpingarea immediately adjacent and surrounding a region wherein gases may beevolved so that substantially no delay results between the evolution ofgas and the removal of same trom the region of interest. In this manner,it is possible to attain a very substantial vacuum within a regionwherein gases are being evolved and also to maintain this substantialvacuum without pressure variations of substantialmagnitude therein. Eventhough relatively large quantities of gas maybe evolved at periodic orsporadic intervals, the very substantial pumping speed which is realizedby the present invention and the immediate proximity of the pumping areato the region of interest herein provides for the immediate removal ofsuch gases to thereby maintain within the region a very substantialvacuum.

Considering now a single preferred embodiment of the present inventionas illustrated in the drawings, and referring to these drawings, therewill be seen to be provided a furnace chamber 11, illustrated in planview in FIG. 1 and containing, for example, a mold 12 into which meltedmetal is adapted to flow during electronbeam casting of same. Thischamber 11 is desired to be mainftained at a high vacuum, as of theorder of 0.01 micron pressure. With the introduction of metal, asindicated by the billet 13, into the chamber and the melting of thisbillet therein, a substantial quantity of gas may be evolved within thechamber. This gas must thus be very rapidly removed in order to maintaina substantial vacuum in the turnace chamber. In the interest of clarityvarious portions of the electron beam furnace are not illustrated in thedrawings of this application but only the mold 12 and billet 13 of metalto be melted are illustrated. The chamber 11 may be defined at thebottom thereof by a solid wall v14- and at the top thereof by a similarwall 16 with the upstanding portion of the chamber between this top andbottom being almost entirely defined by pumping area.

There is herein provided about the periphery of the chamber 11 anddefining same a plurality of strip jet pumps 17 to 28 which form apumping wall of the chamher. The term strip pump will be seen toidentify the physical structure of the pumping means, inasmuch as sameinclude elongated strips. Thus, the structure of the pump 17, ctorexample, includes a pair of vertical strips i 3 1 and 32 extendingupward from and in communication with a boiler, or the like, 33. Each ofthe upstanding strips 311 and 32 consists of a hollow member adapted toconduct a pumping medium such as a vapor from the boiler 83, and each isprovided with slots along the trailing edges of the strip through whichsuch pumping medium is forced. These strip slots are oriented so thatthe pumping medium, such as a vapor, will escape from the strips underpressure generally rearwardly of the strip, as indicated by the arrows'34 in FIG. 1. The high velocity vapor flow, as is established by theescape of pumping medium from the strips 31 and 3 2, will be seen tooperate to entrap or entrain gas molecules in the vicinity of the stripsto thereby move same to the rearof the pump Whereat same may beexhausted. In the present instance there may be provided in connectionwith the rear of each of the strip pumps a conventional backing pump 36,so that the strip pump :17, for example, ex

hausts into the intake of the pump 36 which operates to discharge thepumping medium and entrained gas mole-' cules into the atmosphere aboutthe chamber. The strips 31 and 32 are preferably formed of a very smallcrosssection so as to occupy a of volume while yet establishing arelatively large flow of vapor or the like to accomplish maximumpumping. Suitable connections to the boiler 36 serve to produce thevapor, or the like, employed as the pumping medium in these pumps, as,for example, by the heating of a liquid therein.

The chamber 11 of the present invention is defined by the plurality ofpumps -1728 with common radial walls 37-48 being provided about thecircumference of the chamber 11 between the strips or the separatepumps. These walls 37-48 are vertically disposed in general parallelismwith the strips of the individual pumps and serve to separate theadjacent pumps. The walls are also preferably disposed radially of thechamber 11 about same with an exterionwall 49 being disposed about theouter circumference of these radial walls in attachment .with thechamber top 116 and bottom 14 to thereby complete the chamber structure.I

It will be seen that upon energization of the strip pumps 1728 byoperation of the boilers thereof to establish a desired flow of pumpingmedium through the individual strips of the separate pumps, there willbe produced a pumping action tending to evacuate the chamber 11. Thepumping medium, such as a vapor, which is forced from the boilers of thepumps through the strips thereof to pass outwardly and rearwardly fromthese strips "between the radial walls separating the adjacent pumpswill serve to produce a pumping action of very substantial magnitude. Itwill be noted from the structure illustrated in FIG. 1 in particularthat a very small physical volume is occupied by the front strip 31 ofthe pump 117, for example, and by the edge of the radial walls whichdefine the periphery of the chamber 11. The wall about the chamber lrlis thus substantially defined by pumping area between the front strip ofeach of the strip pumps and the edges of the radial walls disposedbetween the separate pumps. The physical structure of the rear strip ofeach of the pumps is not particularly critical in the present instanceinasmuch as same does not limit the inlet area to the pumps. It ispossible in accordance with the present invention to provide a chamberstructure wherein the upstanding walls thereof are composed almostentirely of pumping area rather than physical structure. In accordanceherewith there have been constructed vacuum chambers wherein the peripheral wall is composed of percent pumping area. Such a vacuum chamber hasfurther been employed as a furnace chamber for electron-beam bombardmentmetal purification wherein a high vacuum is requisite to propercompletion of the purification reaction. A material improvement incasting and purification reactions of this type have been realized withthe vacuum chamber of the present invention inasmuch as it has beenpossible to maintain a pressure of the order of 0.0 1 to 0.1 micron ofpressurein a furnace wherein commercially feasible melting rates areattained. Such a furnace chamber provides a pumping speed of the orderof hundreds of thousands of liters per second, so that greatly increasedmelting rates are possible while yet preventing gas bursts by theextremely rapid removal of gases evolved during the processing.

The foregoing description of a preferred embodiment of the presentinvention is herein set forth as an example of particular structurewherein the basic concepts of the invention are achieved. Alternativestructures are possible in accordance with the invention and thus, forexample, an ordinary diffusion pump may have the top thereof reorientedso that the pump orifices are integral with the pump jets, to therebyimprove the pumping geometry in accordance with the invention. In thismanner it is possible to approach theoretical efliciencies forconventional pumps and to thereby provide for extremely high pumpingrates, so that theremay bemaintained a very substantial vacuum eventhough relatively large quantities of gases are evolved in the evacuatedregion. The present invention will thus be seen from the forc going toprovide a chamber for evacuation which is in substantial part defined bypumping area rather than by structural members. It will be appreciatedthat gas molecules liberated within a chamber at low pressure willrapidly move thereabout and will, in the normal course of events,encounter and rebound from structural elements defining the chamber soas to thereby materially contribute to the pressure within the chamber.Particularly at low pressures is the foregoing of importance, so that arelatively small quantity of gas evolved within a chamber maintained ata low pressure serves to materially increase the pressure of thechamber.

The present invention operates to prevent pressure excursions from theevolution of gas within the chamber by eliminating the great majority ofstructural members against which gas molecules may impinge. Viewed inanother manner, the present invention provides a maximum pump inlet areain closely spaced relation to the particular region or volume desired tobe evacuated and maintained at high vacuum. Consequently, the presentinvention overcomes certain serious limitations of the prior art andprovides for a substantial advancement in the field of vacuum pumping,particularly as same is related to such processes and apparatuses assuffer from the evolution of gas within evacuated regions.

What is claimed is:

1. An improved vacuum chamber comprising a plurality of vacuum pumpsdisposed about a closed path and having inlet apertures the outlinestructures of which being in contiguous array defining a wall of saidchamber, and additional walls engaging the outline structures of saidpump inlets at the ends thereof for completing enclosure of said chamberwhich is thus surrounded by said pumps and adapted to be maintained at avacuum even during evolution of gas within the chamber.

2. An improved vacuum chamber comprising a plurality of strip jet pumpsdisposed in side-by-side relation about a closed path to define achamber wall, said pumps each having a maximized inlet area'with respectto physical structure at one end so that the structural outlines thereofdefine said chamber Wall substantially consisting of the aggregate ofthe structural outlines of the pump inlet areas, and means including atop and a bottom sealingly engaging said pumps at the ends thereof toclose said chamber, said pump inlets extending from the top to thebottom.

3. An improved furnace chamber comprising top and bottom walls connectedby a circumferential vacuum communicating Wall, a plurality of strip jetpumps each including at least one thin elongated strip having rearwardlydirected slots longitudinally thereof and means forcing a pumping fluidmedium into said strips and out of said slots to establish a pumpingaction, said pumps being disposed contiguously between said top 1taining 11 high vacuum during the evolution of gas therein andcomprising a plurality of thin radial walls disposed about thecircumference of a closed path defining the peripheral boundary of saidchamber, at least one source of pumping fluid medium, a plurality ofthin elongated strips disposed parallel to said radial walls with atleast one strip between each of the adjacent walls, said strips havingslots longitudinally thereof along trailing edges thereof with saidslots directed outwardly of said chamber and being connected to saidsource of pumping fluid medium whereby the latter is forcibly ejectedfrom said strips generally outwardly of said chamber, and meansincluding top and bottom walls engaging said radial walls at oppositeends thereof for closing said chamber above and below said closed path,the open spaces defined by the inner ends of said radial walls and saidtop and bottom walls forming the outlet for gas to be evacuated fromsaid chamber.

5. An improved structure for a circular vacuum chamber comprising aplurality of vertically elongated jet vacuum pumps each including a.boiler with at least one vertical st-rip communicating therewith, saidstrip having a thin, hollow cross-section with a curved leading edge andslots along both sides of the trailing edge whereby pumping fluid mediumfrom said boiler escapes rearwardly from said strips, top and bottomwalls defining said chamber, means mounting said pumps about theperiphery of said walls with the leading edge of said strips facing thechamber, and only thin, radial walls forming walls of said pumps andextending between said top and bot-tom walls and separating said stripscircumferentially of the chamber, whereby the upright peripheral wall ofthe chamber is defined by said pumps with a maximized pumpin area aboutthe chamber.

6. An improved vacuum chamber comprising top and bottom walls, and aperipheral vacuum communicating wall connecting said top and bottomWalls and including a plurality of contiguous vacuum pumps having inletopenings of maximized dimension relative to structural area thereofextending between said top and bottom Walls, with said inlet openingsbeing directed inwardly with respect to the vacuum communicating Wall tothereby .provide for maximum pumping speed from the said chamber sodefined.

7. Apparatus according to claim 1 in which the vacuum pumps are disposedabout a circle and have elongated inlet apertures.

References Cited in the file of this patent UNITED STATES PATENTSSimpson Oct. 12, 1954 Winkler Jan. 7, 1958 OTHER REFERENCES

1. AN IMPROVED VACUUM CHAMBER COMPRISING A PLURALITY OF VACUUM PUMPSDISPOSED ABOUT A CLOSED PATH AND HAVING INLET APERTURES THE OUTLINESTRUCTURES OF WHICH BEING IN CONTIGUOUS ARRAY DEFINING A WALL OF SAIDCHAMBER, AND ADDITIONAL WALL ENGAGING THE OUTLINE STRUCTURES OF SAIDPUMP INLETS AT THE ENDS THEREOF FOR COMPLETING